1. Field of the Invention
This invention is directed toward focusing of x-rays, and more particularly directed toward the shaping and optional monochromatizing of x-rays using cooperating mirrors.
2. Background of the Art
Focused and monochromatized x-radiation has numerous applications including high resolution diffraction, protein crystallography, biotechnology, thin film analysis, and micro-focusing for both point-focus laboratory sources and synchrotrons.
Typical x-ray sources emit x-radiation which is not focused or shaped, and which includes a relatively large band width (or wavelength) distribution. A shaped beam can be formed by collimating source emission with mechanical collimators such as slits or shutters. Any radiation not passing through the mechanical collimator is absorbed or scattered, and therefore is lost from the collimated beam.
Two dimensional optics have been used to form a diverted, focused beam of x-radiation. For a given source strength, the intensity of the shaped beam is intensified when compared with a similar beam formed by simple, mechanical collimation. This technique, however, suffers from strong aberrations and mismatched focal lengths. Furthermore, for a given optic, the focal length of the focused beam is fixed, but can be varied by mechanically changing the curvature of the focusing mirror.
Two dimensional optics consisting of multilayer surfaces have been us ed to generate beams of x-radiation falling within a relatively narrow, predetermined wavelengths. Multilayer optics function on the principle of diffraction rather than reflection. For a wavelength, .lambda., of x-rays, a given d-spacing multilayer optic diffracts x-rays at a defined angle. This relationship, known as Bragg's law, equates .lambda.=2 dsin .theta., where .lambda. is the wavelength of diffracted radiation, d is the d-spacing of the multilayer optic, and .theta. is the incident angle and exit angle.
Kirkpactrick-Baez (P. Kirkpactrick and A. Baez, V. 1948, Journal of the Optical Society of America, 38, pp. 766) introduced an optical focusing scheme which independently collimates a beam of x-rays in two dimensions by employing two, two dimensional orthogonal optics in series. The cross coupling of the optic mirrors will be discussed in more detail in a subsequent section of this disclosure. Multilayer optics have also been incorporated in the mirrors. The Kirkpactrick-Baez (K-B) system, therefore, shapes and partially monochromatizes the beam and enhances the beam intensity for a given x-ray source strength. The K-B system suffers from several shortcomings. Both mirrors cannot be installed at an optimum position since the mirrors cannot be coincident as will be subsequently illustrated. The resultant differences in focal lengths lead to decreased beam flux and increased aberration. Since the source and the mirrors are aligned in series, one mirror will collect radiation from the source at a smaller angle. In addition, aberrations of the mirror nearest the source will be larger than the more distant mirror. With a finite source of x-radiation, the K-B system provides a beam with divergence in two directions, producing a rectangular beam cross-section. The diffraction spot from a beam conditioned in this manner tends to be large, decreasing detection resolution by a detector of x-radiation which interacts with a sample exposed to the beam. Increasing the sample to detector spacing distance will improve sample resolution, while decreasing the detector's angle of acceptance. For a given set of mirrors, the focal length of the K-B system is fixed. Focal length can be changed by mechanically varying the curvature, and thus the focal length, of each orthogonal mirror.
A K-B focusing system with two elliptical mirrors in series has been employed to generate a more sharply focused beam approaching a point source. More specifically, for a point source of x-rays, a point source beam can be generated with a K-B system employing elliptical mirrors. For an object, the image will be magnified or demagnified by the system. Since the mirrors are different distances from the sample object, magnification of the beam at the sample object will be different and, therefore, from a rectangular focal spot as will be subsequently illustrated. For given mirrors, the focal length of the system is constant, and can be changed only by varying the curvature of each mirror.
Thathachari (Y. T. Thathachari, 1953, Procedings of the Indian Academy of science, A, 37, pp. 41) proposed a system with two mirrors arranged orthogonally and side-by-side rather than in series. The divergence of a beam reflected by this technique will be the same if parabolic mirrors are used. Magnification will be the same in both directions if elliptical mirrors are used. This eliminates some of the focusing problems of serial K-B mirrors as previously discussed. Multilayer mirrors have also been used with this side-by-side arrangement to further monochromatize the focused beam. Beam flux is increased, surface imperfections in the mirrors have less adverse impact, and beam divergence decreases thereby improving final resolution of radiation interacting with an exposed sample and sensed by a detector system. The system suggested by Thathachari is formed with two optics mounted orthogonally and side by side. This causes some difficulty in aligning the mirrors, increases the fabrication cost, and yields an apparatus which is somewhat susceptible to misalignment or breakage in use in hostile environments. Furthermore, the focal length of the beam generated by this arrangement cannot be varied for a given set of optics.
In summary of the prior art optics used to form a focused beam of x-rays offer variable focusing in a single plane. Prior art optics employing two orthogonal mirrors in series can be varied in focal length, but require two focusing mechanisms so that each mirror can be mechanically distorted as required. Stated another way, if two plane focusing is desired, then two optics and two focusing mechanisms are required. Fixed focus with two optical planes are also available in the prior art in the form of two, orthogonal side-by-side mirrors. Although the side-by-side arrangement offers advantages over the serial mirror arrangement, it does not offer variable focusing.